Do not expose this to the internet (or probably at all). If you do, you are donating your CPU to a random Bitcoin mining pool

simsim is a minimal SSH server that allows anyone to create an account on the machine it's running on. If a client tries to login as a user that does not exist yet, that user is automatically created and the clients public key is added to their ~/.ssh/authorized_keys file. This has the effect that the first client that tries to login as a given user "claims" that username.

The obvious implication is, that anyone who can connect to your machine, also gains access to it. Hence above warning - obviously this is horrificly insecure and you shouldn't use it on a machine you care about (at all).

The intended usecase is for a friendly programming competition - participants can upload their solutions by logging into a shared machine, which contains binaries that run a prepared test suite against them. Because it's just in good fun, we don't care about what horrible, horrible things people might do to that machine with arbitrary code execution.

• I wrote this in approximately six hours so it's horrificly buggy. I assume that it will break on first contact with the enemy (=user).
• It intentionally only supports public key authentication and for now only ed25519 (though I plan on changing that).
• It so far only supports executing a shell. No port forwarding, no X11, not even running a specific command. As this is not supposed to be production-grade software, that probably won't change.
• It's not very debuggable. Error messages are unhelpful and there are no tests.

simsim relies heavily on the Go x/crypto packages, written and maintained by the Go team, as well as Keith Rarick's pty package.

Copyright 2018 Axel Wagner

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